Various electromagnetic (EM) techniques exist to perform surveys of a subterranean structure underneath a surface for identifying subterranean elements of interest. Examples of subterranean elements of interest in a subterranean structure include hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One survey technique is the magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an EM transmitter, called a “source,” is used to generate EM signals. With either survey technique, surveying units, called “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structures can be derived. The receivers may include a number of sensors for detecting any combination of electric fields, electric currents, and magnetic fields.
A major issue associated with CSEM surveying for detecting thin resistive targets at depth in a subterranean structure is the so-called “air-wave effect,” which is caused by the air-water interface. The air-wave effect is caused by an EM signal portion generated by an EM transmitter that follows a path extending upward from the EM transmitter, through the air, and then back vertically downward through the body of water to an EM receiver. This air-wave signal component is insensitive to thin resistive targets at depth in a subterranean structure and can interfere with accurate characterization of the subterranean structure when processing survey measurement data.